The present invention relates to a control device for cyclic sewing machine, and more particularly to an improvement of a control device for controlling driving condition of a cyclically operated sewing machine.
There is known a cyclic sewing machine which initiates its cyclic sewing operation by a start control manipulation and stops in a predetermined position after a predetermined sewing operation has been performed.
One conventional control device for a cyclic sewing machine is disclosed in Japanese Patent Publication No. 45-22697 (published July 31, 1970). In such Conventional control device, a clutch mechanism for controlling the driving condition of a main shaft comprises a plurality of pulleys, two driving belts each having circular cross-section mounted on the pulleys and a lever for moving the driving belts.
However, in the conventional device, since switching of the main shaft between the high-speed drive condition and the low-speed drive condition are effected by forcibly moving the driving belts with the lever, thus the switching cannot be effected stably and reliably. Further, if the belt having circular cross-section is used, the belts cannot transmit large amounts of power to the main shaft due to shortage of friction between the belt and the pulleys.
The control device for a cyclic sewing machine employing another type of a clutch mechanism has heretofore been proposed to overcome the above-described drawbacks in Japanese Patent Publication No. 56-13480 (published Mar. 28, 1981) corresponding to U.S. Pat. No. 3,908,568. In this conventional control device, the clutch mechanism comprises a pulley, disk-like clutch plates disposed coaxially with the pulley, a flat belt mounted on the pulley and a switching lever for switching engagement condition between the pulley and the clutch plates.
In this device, since the pulley engages the clutch plates stably, the switching can be effected stably and reliably.
On the other hand, a cam for controlling the switching lever which is rotatable at a reduced speed in interlinked relation to the main shaft has a stop control recess defined in an outer periphery thereof, a low-speed control cam surfaces disposed on each side of the stop control recess and positioned radially outwardly of the stop control recess, and a high-speed control cam surface positioned radially outwardly of the low-speed control cam surfaces. The cam and the switching lever are interlinked through a link member of which a roller is urged to always contact the cam surfaces by a spring. In response to operation of a treadle, an operating lever is turned against the biasing force of the spring to displace the roller on a link member out of the stop control recess, and at the same time the switching lever is turned into a low-speed control position against the biasing force of the spring. The turning movement of the switching lever operates a clutch mechanism to connect the main shaft to a low-speed drive source, thus starting to rotate the main shaft at low speed. When the cam is rotated upon the rotation of the main shaft, the roller on the link member is successively brought into engagement with the high-speed control cam surface and the low-speed control cam surfaces according to the cam profile. Upon the engagement of the roller with these cam surfaces, the switching lever is moved into a high-speed control position in which the main shaft is coupled to a high-speed drive source, after which the switching lever is returned to stop control position via the low-speed control position.
It has found that the conventional control device for sewing machine of the above structure suffers from the following problems.
The operating lever is directly subjected to the biasing force of the spring. When an operator steps down the treadle, the operating lever is turned against the biasing force of the spring. Therefore, the spring having the powerful biasing force cannot be used because of necessity of lightening the operator's burden. This causes the roller on the link member to disengage from the cam surfaces temporarily. As a result, when the main shaft of the sewing machine is stopped, there is developed resistance to the return of the roller on the link member from the high-speed control position to the stop control position under the resiliency of the spring, so that the roller moves with a delay in following up the different cam surfaces. Therefore, the timing of switching the main shaft from the high-speed drive source to the low-speed drive source is delayed, and the transmission of power to the main shaft is cut off before the rotational speed of the main shaft is sufficiently reduced. A stopper then violently hits a stop groove defined in a stop member fixedly mounted on the main shaft, components of the sewing maching tend to be damaged. Further, a stopper tends to bounce out of the stop groove whereupon the main shaft is stopped, the sewing needle of the sewing machine is liable to stop in different positions.
To avoid the above drawbacks, the cam profile may be changed so as to prolong the low-speed period, to thereby advance the timing of switching the main shaft from the high-speed drive source to the low-speed drive source. This solution however causes another problem that the cyclic period of time of the sewing machine is longer, since the time of the low-speed condition is longer.
Still another conventional control device for a sewing machine is described in U.S. Pat. No. 3,894,500.